1. Field of the Invention
The present invention relates to a liquid-crystal lens and method and apparatus for driving the same.
2. Description of the Related Art
Conventionally, a variable focal-length lens using a liquid crystal is known. The liquid-crystal lens has a structure in which a liquid crystal provided between transparent electrodes is formed on a glass substrate having the shape of a lens. In the liquid-crystal lens, the liquid crystal is changed in refractive index by changing the application voltage to the transparent electrodes, thereby electrically changing the focal length as a lens. However, the thickness of the liquid-crystal layer must be increased to achieve a sufficient change in focal length. This results in a drawback of requiring a large response time in reaching a desired focal length from a voltage application, in addition to the difficulty encountered in fabrication. This is because response time is inversely proportional to the square of the liquid-crystal layer thickness.
Meanwhile, there is another kind of the variable focal-length lens provided with a liquid crystal arranged between transparent electrodes on a glass substrate having the shape of a hologram lens, for example, disclosed in Japanese Patent Kokai No. 5-34656. In this case, however, variable focal length is available only where the phase step of hologram lies corresponding to integer times a wavelength of use. The intermediate voltage if applied to the liquid crystal causes deterioration of transmission efficiency (hologram diffraction efficiency).